1. Field of the Invention
The present invention relates to code assignment in multi-rate communications of the radio communication system employing Code Division Multiple Access (CDMA) system, where codes having tree structured orthogonal multi-spreading factor sequence are used as spreading codes.
2. Description of the Related Art
The system utilizing coherent multi code and the system utilizing tree structured multi-spreading factor sequence code have been introduced as a multi-rate transmission system in the radio communication system employing DS-CDMA (Direct Sequence-CDMA). In the multi-rate transmission utilizing the coherent multi code, plural codes are simultaneously used to perform multi-code-multiplexing. In the multi-rate transmission utilizing the tree structured multi-spreading factor sequence code, orthogonal codes having equivalent spreading factors are multiplexed in parallel. Receivers for receiving plural codes are needed for a mobile station used for the multi-rate transmission utilizing the coherent multi code. On the other hand, only one receiver is needed for the system utilizing the tree structured multi-spreading factor sequence code.
The conventional CDMA system using tree structured orthogonal multi-spreading factor sequences is disclosed in, for example, a technical report entitled “Multi-rate Wideband DS-CDMA Radio Access For Next Generation Mobile Radio Systems”, the Institute of Electronics, Information and Communication Engineers, RCS-97-86, 1997. The conventional system will be explained with reference to FIG. 11.
FIG. 11 shows a configuration of the conventional tree structured orthogonal multi-spreading factor sequences.
The tree structure is composed of four levels: level 1 at the top through level 4 at the bottom, defined just for convenience. As there are four levels in this case, the channel transmission rate using the top level (level 1) code is eight times as fast as that of the channel using the lowest level (level 4) code. The channel transmission rate using level 2 code is four times the rate of channel using the level 4 code. The channel transmission rate using level 3 code is twice the rate of channel using the level 4 code. Cx(y) in FIG. 11 represents a kind of codes, x stands for a code level, y stands for an identification number in the same code level, and .smallcircle. stands for a code.
In FIG. 11, a code in the level 1 can be used for communication of twice the transmission rate of level 2. Now, a concrete example will be explained referring to the case of tree structure as shown in FIG. 11. When C.sub.2(1) is assigned to a user of four times the transmission rate of the lowest rate (C.sub.4(x) level in FIG. 11), the codes C.sub.4(1), C.sub.4(2), C.sub.4(3), C.sub.4(4), C.sub.3(1), and C.sub.3(2) can not be assigned to other users because of keeping the code orthogonality. Due to this characteristic, all the lower level codes connected with an assigned code through branches and all the upper level codes connected with the assigned code through only upward branches are not-assignable. When C.sub.2(1) is the assigned code, all the lower codes C.sub.4(1), C.sub.4(2), C.sub.4(3), C.sub.4(4), C.sub.3(1) and C.sub. 3(2) connected with the assigned code by branches are not-assignable, and the upper code C.sub.1(1) connected with the assigned code by the shortest branch is not-assignable, either.
As stated above, in CDMA system using codes of tree structured orthogonal multi-spreading factor sequences, the code assignment for a mobile station is restricted because of the characteristic. Accordingly, in the case of FIG. 11, if C.sub.4(5) and C.sub.4(7) are assigned to calls of the lowest rate (level 4), it is impossible to assign a call having the rate of level 3 or upper than level 3. However, on the supposition that C.sub.4(6) is assigned to the call of level 4 instead of C.sub.4(7), it is possible to assign C.sub.3(4) to the call of level 3, consequently channels are effectively utilized.